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NFPA 820 Vented Boundary Between Class 1 Div 1, Div 2, and Unclassified

NFPA 820 Vented Boundary Between Class 1 Div 1, Div 2, and Unclassified

(OP)
I have a sewage pump lift station which I am working on, and trying to determine the boundary locations from Class 1 Div 1, to Class 1 Div 2, to Unclassified.

NFPA 820 Figure A.4.2(c), as well as several other figures, shows a vent to atmosphere. Inside the wet well is a Class 1 Div 1 space. Within 3 ft of the vent is Div 1, from 3 to 5 feet is Div 2, and outside 5 feet is unclassified. This figure refers to Table 4.2.2 rows 16 and 34.

My question is: where are the 3 and 5 foot boundaries defined in the table or in the text of the code? The nearest thing I can find that classifies the radius around a vent is Table 4.2.2 row 20, which seems to only deal with Division 2 classifications and does not say anything about a 5-foot radius. I also couldn't find these boundaries in the NEC, except for article 500.5(A)(2)(3) which says that a Class 1 Div 2 space exists adjacent to a Class 1 Div 1 space.

Looking for help in determining where these boundaries are defined...Is it just based on the images alone? I would think there should be corresponding text in a table or paragraph.

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